The flame structure and soot formation in the counterflow Ethylene-Air nonpremixed flame are numerically analyzed. The present soot reaction mechanism involves nucleation, surface growth, particle coagulation, and oxidation steps. The gas phase chemistry and the soot nucleation, surface growth reactions are coupled by assuming that the nucleation and soot mass growth has the certain relationship with the concentration of benzene and acetylene. In terms of the centerline velocity and the soot volume fraction, the predicted results are compared with the experimental data. The detailed discussion has been made for the sensitivity of model constants and the deficiencies of the present model. Numerical results indicated that the acetylene addition to the soot surface plays the dominant role in the soot mass growth for the counterflow nonpremixed flame.